1. Field of the Invention
The present invention relates to a method for producing a preform, which is substantially free of OH impurities, for an optical fibre, wherein one or more quartz layers, which may or may not be doped, are deposited on the internal surface of a quartz glass support tube, in which method a furnace is moved axially with respect to the support tube, and after the quartz layers have been deposited the support tube is contracted into a bar-shaped preform while being heated. The present invention furthermore relates to a fibre obtained from such a preform.
2. Description of the Related Art
The method as referred to in the introduction is known from German Offenlegungsschrift No. 37 31 346. According to the method disclosed therein, a quartz glass support tube is internally coated according to the so-called MCVD method (Modified Chemical Vapour Deposition) with one or more doped and/or undoped glassy quartz layers which correspond to the core and/or the cladding of the quartz optical light waveguide to be formed. Said internal coating takes place in such a manner that a glass-forming mixture, SiCl4, for example, and also O2, is introduced into said support tube on one side thereof and so heated therein, for example to a temperature of 1600xc2x0 C., that the desired glass layer is deposited on the internal surface. The heat source required for that purpose is for example a flame which is moved axially with respect to the support tube. Furthermore it is known from said German Offenlegungsschrift not to rotate the quartz glass support tube. This method of non-rotation is in particular applied here because the rotary seals which are consequently required are not needed upon introduction of the glass-forming gases, since such rotary seals would have the drawback of a disruptive water vapour, for example formed from the humidity in the air, finding its way into the support tube in an uncontrollable manner in case of inadequate seals.
The burning process and the high temperatures may thus lead to the inclusion of the undesirable OH groups in the external surface of the support tube. Since the support tube forms part of the fibre yet to be drawn, it is to be expected that said included OH groups will present problems as regards the optical properties of the glass fibre that is eventually obtained. The OH groups that are included on the outer side can diffuse inwardly, that is, in the direction of the core, in the course of the further processing steps, when temperatures are high. It has become apparent that said OH groups produce adverse effects in the light-conducting part of the optical fibre. After all, the OH groups exhibit a wide absorption peak at 1385 nm. As a result, additional signal loss occurs in the optical glass fibre with the transmission wavelengths around 1300 nm and 1500 nm that are currently being used. Moreover, said absorption peak at 1385 nm limits the use of the fibre over a large wavelength range that recent developments require. Another drawback of the included OH groups on the external surface of the support tube is the fact that they limit the reduction of the portion of CVD (chemical vapour deposition) glass in the optical fibre.
Thus it is desirable that the effect of the inwardly diffusing OH groups is minimized, thus minimizing the signal loss at the standard transmission wavelengths, which makes the fibre very suitable from a commercial point of view.
Moreover, the prior art heat sources, viz. the use of the flame technique, exhibit an additional drawback in that expensive combustion gases are used, with a relatively great deal of energy being lost. In addition, a rather considerable amount of glass is lost as a result of vaporizing in the flame, and the contraction speed is limited.
Another method for making a preform which is substantially free of OH impurities is known from U.S. Pat. No. 5,397,372. According to the method disclosed therein, a vapour mixture including at least one compound glass-forming precursor is introduced into a (possibly rotating) support tube together with an oxidizing medium, generating a hydrogen-free isothermal plasma on an outer surface of the tube as the heat source, as a result of which the glass-forming mixture will react and a glassy deposit will be produced on the inner surface of the tube, thereby forming a glass preform.
A similar method is known from European patent application No. 0 171 103, wherein a plasma furnace is moved round a rotating support tube along the axial length thereof so as to produce a preform. No mention is made of the presence of harmful OH impurities.
German Offenlegungsschrift No. 40 01 462 discloses a method for producing a preform wherein one or more layers of glass are deposited on the internal surface of a vertically clamped-down support tube. The heat source that is used is a graphite resistance furnace, which is moved axially over the non-rotating support tube. No mention is made of the presence of harmful OH impurities.
The method of moving a furnace axially with respect to the support tube, wherein the support tube rotates in the furnace, is known per se from U.S. Pat. No. 5,090,978, wherein a torch assembly is used as the heat source. No mention is made of the presence of harmful OH impurities.
U.S. Pat. No. 4,009,014 discloses a method for collapsing a layered support tube into a bar-shaped preform by using heat, which method uses an oxyhydrogen flame as the heat source.
European patent application No. 0 140 113 discloses a method for manufacturing optical fibre preform wherein the support tube is heated while being rotated, using a stationary heat source of length equal to the support tube. Such a heat source is controlled in such a manner that a temperature profile is applied both over the radius and along the length of the support tube in order to thus enhance productivity and reproducibility; no mention whatever is made as regards preventing the harmful presence of OH impurities.
Accordingly, it is an object of the present invention to provide a method for producing a preform which is substantially free of OH impurities, which method eliminates the drawbacks of the prior art.
It is another object of the present invention to produce a preform containing a low amount of OH impurities by using a quick and inexpensive process.